(a) Field of the Invention
The present invention relates to an optical recording medium and, more particularly, to an optical disk for recording/reproducing thereon information by irradiating a laser beam onto a recording film to change the optical characteristic of the recording film.
The present invention also relates to an optical read-only-memory (ROM) disk including a super-resolution optical ROM film from which data are reproduced using a super resolution technique.
(b) Description of the Related Art
In general, rewritable optical disks, such as magneto-optical disk and phase change disk, are used for recording/reproducing information by irradiating a laser beam onto a recording film to thereby change the optical characteristics thereof such as the magneto-optical characteristic, reflectance and optical phase. Optical disks have been required to have a higher recording capability, and now achieved a 16-fold-speed recording scheme as in the DVD-R and DVD-RAM. The optical disks are also required to have a larger recording capacity in addition to the higher-speed recording capability.
For increasing the recording capacity of an optical disk, it is effective to record/reproduce the information by using a small-diameter laser beam. Thus, vigorous researches have been conducted on the study of optical disks on which the information is recorded/reproduced using a laser beam having a wavelength of around 405 nm. Such an optical disk generally uses a reflecting film configured by a material including Ag as a main component thereof. This is because the Ag can be obtained at a lower cost compared to the other precious metals, and has a higher reflectance at a wavelength range around 405 nm and a higher thermal conductivity, thereby achieving a higher-speed recording and yet allowing a higher signal amplitude in the reproduced signal.
In general, the phase change optical disk has a configuration wherein a first dielectric film, a recording film, a second dielectric film and a reflecting film are consecutively deposited on a substrate. These dielectric films may be formed as a multilayer dielectric film, if desired. A so-called interface film may be additionally formed between the dielectric film and the reflecting film for improving the crystallization rate or increasing the number of overwrite times. A ZnS—SiO2 film including mixture of ZnS and SiO2 is widely used as the dielectric film in the phase change optical disk. The ZnS—SiO2 film has a refractive index as high as around 2.2, and a higher sputtering rate suited to a mass production. In general, a ZnS—SiO2 dielectric film having a ZnS content of 50 to 80 mol % is widely used, and a composition of (ZnS)80—(SiO2)20 is most popularly used, the (ZnS)80—(SiO2)20 including ZnS at a rate of 80 molar percent (mol %) and SiO2 at a rate of 20 mol %.
It should be noted here that Ag is liable to sulfuration and thus, arrangement of the reflecting film including Ag adjacent to the dielectric film including ZnS—SiO2 in the phase change optical disk necessitates the scheme of preventing sulfuration of Ag. Patent Publication JP-2004-111016 describes an optical disk having a sulfuration suppressing film, such as including SiC, SiN and GeN.
Use of the sulfuration suppressing film increases the filming step in the manufacture of the phase change optical disk to decrease the mass production rate of the optical disk, however. There is no known technique to suppress the sulfuration of the Ag-containing reflecting film in the mass production of the optical disk without using such a sulfuration suppressing film.